1. Field of the Invention
This invention disclosure relates to petroleum exploration and production, a method of tar/tar sand prediction to reduce cost and risk of oil exploration and production.
2. Discussion of Related Art
This section of this document is intended to introduce various aspects of the art that may be related to various aspects of the present invention described and/or claimed below. This section provides background information to facilitate a better understanding of the various aspects of the present invention. As the section's title implies, this is a discussion of related art. That such art is related in no way implies that it is also prior art. The related art may or may not be prior art. It should therefore be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art.
Tar is a dark brown or black bituminous usually odorous viscous liquid obtained by destructive distillation of organic material. It constitutes the heavy end of hydrocarbon with high content of asphaltenes. In one form, tar and residue oil are found in petroleum reservoirs. These tars often affect oil productivity due to their low permeability. Tars are also found in the porous sands near fault zones or in the vicinity of base of salt. These tars or tar sands are usually pressurized and some of them are mobile once being penetrated by drilling. As the viscous tar fluid flows into the borehole, it may be very difficult to drill through, thus, causing serious problem for drilling and often resulting loss of operation time and money.
Tars are present in many oil reservoirs throughout the world. They are quite common in carbonate reservoirs in the Middle-East, including several of the ten largest reservoirs in the world. Tars are usually—but not always—located at or near present-day oil/water contacts. The tar saturation is commonly unpredictable, at least at the level of detail needed to effectively place injection wells to minimize loss of mobile oil above the tar and to minimize the risk of sidetracking wells to get back above the tar.
To gain full control of these issues, many questions need to be answered: where are the tars (tar sands, tar mats) located? How many are there? What is their horizontal distribution? Do they constitute strong permeability barriers or only partial ones? Can we predict and model tar distribution in a reservoir? Placing horizontal injectors just above tar may require specific real-time geosteering technology.
Numerous cases of tar encounters during petroleum drilling were reported in the deepwater of Gulf of Mexico (Rohleder et al., 2003; Romo et al., 2007; Weatherl, 2007; Gang Han et al., 2008). The tar problem has become a technical challenge and economic risk for the deepwater drilling especially in the Gulf of Mexico. For this reason, extensive studies have been done on the physical property and diagnostic signature of tars/tar sands. However, technique for seismic-based pre-drill (prior to drilling a well) prediction is rare. In fact, some authors even indicate that the “presence of tar cannot be predicted with current seismic technology and there is not a recognized seismic signature” (Romo and others, 2007).
There is an acute need for the industry to find a way to predict the “unpredictable” tar or tar sands on a pre-drilling basis such that drilling through tar can be avoided.